EP0465697A1 - Preparation of gas separation membrane and spiral wound gas separation module - Google Patents

Abstract

The invention relates to a process for preparing gas separation membranes or modules. Membrane forming material is cast into a wet membrane which is wound into a spiral module (88). The spiral module (88) is arranged in a closed circulating apparatus (90). Exchanging solvents or treating solutions are circulated through the circulating apparatus (90) the spiral module (88) for an appropriate period of time. The module and circulating apparatus are purged until the membrane is dry, thus producing a gas separation membrane or module.

Description

BACKGROUND OF THE INVENTION
• The invention relates to a process for preparing gas separation membranes and modules. Gas separation membranes are films which exhibit a higher permeability toward some gases than toward others. Industries increasingly employ such membranes for such uses as: oxygen enrichment of air, purification of nitrogen, recovery of hydrogen from hydrogen reaction systems, removal of acid gases from natural gas or biogas, and separation of acid such as Co₂ from H₂S.
• Such membranes have no moving parts, negligible maintenance, low energy requirements, operational flexibility, and rapid start up and shut down. They may be formed as flat sheets, hollow fibers or tubes. A reinforcing fabric of woven or nonwoven polyester, or nylon increases membrane strength, improves operational properties and provides longer life.
• The rate at which gas permeates a membrane can be expressed as p/I, where p is the gas permeability, and I is the effective membrane thickness. Cellulose acetate exhibits particular commercial importance because its effective thickness is about 0.1 micrometers. This in turn, results in a relatively high gas permeation rate.
• Cloth reinforced membranes have been prepared by casting a cellulose acetate solution on the cloth and then gelling, leaching, annealing, solvent exchanging and drying the cloth/membrane combination. US-A 3 497 072 discloses a phase inversion technique for casting a cellulose acetate membrane. If a wet membrane is to be dried directly, the high surface tension of water will destroy the microporous structure. Drying methods which can prevent this destruction include first freeze drying; second adding an agent to reduce surface tension; and third exchanging the water solvent for another solvent. US-A 3 842 515 describes a solvent exchanging process which exchanges water with water missible alcohol, then exchanges the alcohol with a nonpolar organic solvent, and finally evaporates the nonpolar organic solvent to leave a dried membrane. US-A 4 080 743 describes a solvent exchanging process which first exchanges water with water missible organic solvents, and then evaporates the mixture to leave a dried membrane.
• During the step of drying, different shrinkage ratios between the cellulose acetate membrane and the reinforcing cloth causes curling, wrinkling and cracking. Moreover, brittle membranes sometimes crack during winding into spiral wound modules. US-A 4 134 742 discusses these problems and attempts to reduce curling by utilizing a reinforcing cloth having a shrinkage ratio close to that of the membrane. This approach does not solve problems caused by uneven shrinking between the membrane in the reinforcing cloth, nor does it reduce cracking problems when winding a dried membrane into spiral wound module.
DISCLOSURE OF THE INVENTION
• It is the primary object of the present invention to provide a novel and improved process for forming gas separation membranes and spiral wound gas separation modules avoids shrinking and curling by winding wet membrane onto a core before drying.
• The scope of the invention is defined in the appended claims. It is shown that the spiral wount wet membrane is placed in a treating container, and treating solutions for solvent exchange are circulated through the container and membrane. After solvent exchange, the spiral wound membrane is dried.
• In the following the invention is described more detailed in connection with the embodiments of the invention shown at the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
• Figure 1 is a partially cut-away view of a spiral wound gas separation membrane;
• Figure 2 is a flow chart of steps in a former process for producing spiralwound gas separation membranes and modules;
• Figure 3 is a schematic representation of an apparatus using the process of Fig. 2;
• Figure 4 is a flow chart of steps in a process for producing spiralwound gas separation membranes and modules embodying the present invention; and
• Figure 5 is a schematic representation of an apparatus for treating and drying gas separation membrane modules in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• The invention is a process for producing a gas separation membrane and module. Materials used for industrial gas separation membrane applications include cellulose acetate (CA), polysulfone, polyamide, polyimide, polycarbonate, polysiloxanes, poly (4-methyl-1-pentene), poly (phenylene oxide) and poly (vinylidene fluoride).
• Figure 1 shows an example of such a spiral woundgas separation membrane 11 as disclosed in US-A 4 134 742. The module comprises amultilayer membrane sheet 13 wound onto ahollow core member 15 havingperforations 17. Themultilayer membrane sheet 13 consists of two layers of cloth reinforcedmembrane 19 separated by a layer oftricot netting 21. Each layer of reinforcedmembrane 19 is covered by an exterior layer ofvexar netting 23 on the opposite side of themembrane 19 from thetricot netting 21. In operation, a gas mixture is feed into theinput end 25 of the spiral woundmultilayer membrane sheet 13. The gas mixture spreads through thevexar netting 23, and the more permeable gas of the mixture penetrates the reinforcedmembrane 19 into thetricot netting layer 21. The tricot netting 21 conducts the selected gas spirally toperforations 17 of thehollow core member 15. The permeated gas then passes through theperforations 17 in thehollow core member 15 and can be removed through thepermeate output 27 of thehollow core member 15. Non-permeable gas of the gas mixtures passes through thevexar netting 23 and exits through theresidual output 29.
• Figure 2 shows aprocess 35 descriped in US-A 4 134 742 for producing a spiral wound gas separation membrane as shown in Figure 1. In a casting step 37 wet membrane is cast onto a reinforcing fiber. In apost-treating step 39, the wet membrane is treated by gelling (immersing in a cold liquid), annealing (immersing in a hot liquid), or other treatment. In asolvent exchange step 41, one or more solvents replaces water used in the cast material. In adrying step 43, the exchange solvent is dried from the membrane. In awinding step 45, the dried membrane is wound onto a core to produce a spiral wound module.
• Figure 3 shows an apparatus for forming a spiral wound gas separation module using the traditional process of Figure 2. Acasting solution 51 is cast onto a reinforcingfiber 53 to form a reinforcedwet membrane 55. The reinforced wet membrane is immersed in agelling tank 57 which contains cold water. The gelled membrane is then immersed in anannealing tank 59 containing hot water. The annealed membrane is then immersed in one or moresolvent exchange tanks 61 which contain a series of exchange solvents. After solvent exchange, the membrane is dried in adrying apparatus 63. The driedmembrane 65 is then wound onto a core to form aspiral wound module 67.
• Figure 4 depicts the steps of the process of the present invention in a flow chart form. In acasting step 77 wet membrane is cast onto a reinforcing fiber. In awinding step 79 the fiber reinforced wet membrane is wound into a spiral module. In a treatingstep 80, treatment solutions may be circulated to gel, anneal, repair or accomplish other changes to the particular membrane material used. In asolvent exchanging step 81 solvents replace water in the spiralwound module. Solvents are typically organic solvents. In adrying step 83 the solvents are evaporated to leave a dry membrane.
• Figure 5 depicts an apparatus for conducting solvent exchange and drying of the module. Aspiral module 88 is placed in acontainer 90 having aninlet 92 and anoutlet 94 for fluid. An O-ring, or U-cup 97 forms a seal between themodule 88 and thecontainer 90 so that any fluid passing from thecontainer inlet 92 to theoutlet 94 must pass through thespiral module 88. A circulatingpump 99 transfers fluid from areservoir 101 to thecontainer inlet 92. Excess fluid exits thecontainer 90 through thecontainer outlet 94 and returns to thereservoir 101. After completing solvent exchange, thefluid pump 99 can be replaced with anair pump 103 and thecontainer outlet 94 can be connected to avacuum pump 105. Theair pump 103 forces dry air into thecontainer 90 and thevacuum pump 105 assists removal of air from the container through thecontainer outlet 94. Thevacuum pump 105 is optional. Thereservoir 101 may contain multiple sections for storing different solvents. The reservoirs may be temperature controlled, for example, to provide a cold gelling solution, a hot annealing solution, or a cold exchange solvent. The reservoir may hold a treating solution of the same material used for casting the wet membranes, thereby repairing damaged membranes. The embodiments described below illustrate details of the present invention.
Embodiment 1
• A cellulose acetate cast solution made up of materials listed in the Table 1 was cast on a glass plate with a 10 mill doctor blade and allowed to gel for 10 to 15 minutes. The solution contains both organic and inorganic materials.

  
• The membrane was removed from the glass plate and annealed by emersion in 86 centigrade water. Annealed membrane was then immersed in isopropanol for 7 minutes, then in n-heptane for 7 minutes and then air dried at ambient room temperature. Polyester cloth was separately immersed in 86° centigrade water, immersed in isopropanol and n-heptane for 7 minutes each, and air dried in the same manner as the membrane. Shrinkage for both the membrane and the polyester cloth were measured and the results are tabulated in Table 2.

  
• Because of the high shrinkage difference between membrane and polyester cloth, membrane reinforced with polyester cloth would curl, especially during the drying step.
Embodiment 2
• The cellulose acetate solution of embodiment 1 was applied to polyester cloth with a 10 mill doctor blade, held for 5 to 10 second, then gelled in water at 1 centigrade for 5 to 15 minutes. The reinforced membrane was immersed in isopropanol at 3 centigrade for 7 minutes, then in n-heptane at 3 centigrade for 7 minutes. The reinforced membrane was fixed between two plates having a central hole so as to hold it around its periphery. A treating solution of 0.2% silicone and n-heptane was then poured onto the upper side of the reinforced membrane and a vacuum was applied to the lower side for 10 minutes. Application of the treating solution was stopped and vacuum held until the membrane dried completely. Gas permeation rates and selectivities of the dried membrane under 100 psi pure gas were measured and the results tabulated in Table 3.

  
Embodiment 3
• A wet cast reinforced membrane identical to the one used in embodiment 2 was prepared in an identical manner, except that, soaking in isopropanol and n-heptane occurred at room temperature. Gas permeation rates and selectivities of the resultant dry membrane were measured and are tabulated in Table 4.

  
• The results revealed that the membrane exhibits good performance even though solvent exchange occurred at room temperature.
Embodiment 4
• A cast solution made up of materials listed in Table 5 was cast on polyester cloth with a 10 mill doctor blade, then gelled in 20 centigrade water for 10 to 20 minutes.

  
• The reinforced membrane was fixed between two plates having central holes so as to hold it around its periphery and dried at 30° centigrade, 50% relative humidity for 24 hours. A 0.5% silicone solution in n-heptane was poured on the dry membrane while applying the vacuum on the lower side for 1 minute. The solution was stopped and vacuum applied until the membrane dried completely. Gas permeation rates and selectivities were measured, and the results are tabulated in Table 6.

  
Embodiment 5
• The cast solution of embodiment 2 was cast on polyester cloth and held for 5 to 10 seconds. The reinforced membrane was then gelled at 1 centigrade for 5 to 15 minutes. The membrane was then heat annealed at 89° centigrade for 5 minutes, after which it was immersed in isopropanol at 3° centigrade for 7 minutes and n-heptane at 3° centigrade for 7 minutes. The membrane was air dried at ambient temperature, and observation showed severe curling as illustrated in Figures 5 and 6. Gas permeation rates and selectivities were measured and the results are tabulated in Table 7.

  
• This data reveals that cracking and curling causes lower selectivity.
Embodiment 6
• A 40 inch wide membrane was prepared by continuous type membrane coating and wound into a 4 inch by 40 inch spiral module. The module was placed in a container as shown in Figure 6. Isopropanol was circulated for 1 day, then n-heptane for 1 day, then a 0.1 % silicone solution in n-heptane for 1 day. Excess solution was decanted from the container, and the module was purged with dry air from the container inlet while applying vacuum to the container outlet. Gas permeation rates and selectivities of the dried module were measured under 100 psi pure gas, and the results are tabulated in Table 8.

  
Comparative Example
• Two 2 inch by 12 inch annealed gas separation modules were prepared, one by drying before rolling and the other by rolling before drying. The selectivities of each membrane to nitrogen and oxygen were measured and the results shown in the first line (sample 1) of the Table 9. Two 2 inch by 12 inch unannealed membranes were similarly prepared, one by the drying before rolling and one by rolling before drying. The selectivities of the unannealed membranes to nitrogen and oxygen are shown in the second line (sample 2) of Table 9.

  
• It is to be realized that only preferred embodiments of the invention have been described and that numerous substitutions, modifications and alterations are permissible without departing from the spirit and scope of the invention as defined in the following claims.

Claims (21)

1. A process for preparing gas separation membranes and modules, characterized in the steps of:

- casting a solution of membrane-forming material into a wet membrane,

- winding the wet membrane into a spiral module,

- arranging the module in a treating apparatus,

- circulating a treating solution through the module for an appropriate period of time, and

- purging the module until the membrane is dry.

2. A process according to claim 1 for preparing spiralwound gas separation modules, characterized in the steps of:

- casting a solution of membrane-forming material into a wet membrane,

- winding the wet membrane into a spiral module,

- arranging the spiral module in a unidirection flow treating container,

- circulating a treating solution for an appropriate period of time through the module and container, and

- purging the module and container until the membrane is completely dry.

3. A process according to claim 1 for preparing composite membranes characterized in the steps of:

- casting a solution of organic and inorganic membrane-forming material into wet membrane on a reinforcing substrate,

- winding the reinforced wet membrane into a spiral module,

- arranging the module in a unidirectional-flow treating container,

- circulating a treating solution for an appropriate period of time through the module, and purging the module and container with gas until the module is dry.

4. A process according to claim 1 and 2, characterized in that the membrane forming material comprises a combination of organic and inorganic materials.

5. A process according to claim 1 to 3, characterized in that the organic membrane forming material comprises a natural macromolecule.

6. A process according to claim 1 to 3, the organic membrane forming material comprises a synthetic polymer.

7. A process according to claim 1 to 3, characterized in that the organic membrane-forming material comprises one of cellulose acetate, polysulfone, polyamide, polyimide, polycarbonate, polysiloxane, poly(4-methyl-pentene), poly (phenylene oxide), and poly(vinylidene fluoride).

8. A process according to claim 1, characterized in that the treating apparatus comprises:

- a closed container (90) having a container inlet (92) and a container outlet (94), and

- a seal (97) disposed within the container (90), so that when the spiral module (88) is disposed within the treating apparatus, fluid pumped into the inlet (92) passes through the module (88) to the outlet (94).

9. A process according to claim 1 to 3, characterized in that the treating solution comprises a solution of the membrane forming material used for casting the membrane or the wet membrane.

10. A process according to claim 1 to 3, characterized in that the treating solution comprises an organic solvent.

11. A process according to claim 1, characterized in that the treating solution comprises silicone.

12. A process according to claim 1, characterized in that the period of circulation of the treating fluid is in the range of 1 minute to 48 hours.

13. A process according to claim 1, characterized in that the period of circulation of the treating fluid is in the range of 1 hour to 24 hours.

14. A process according to claim 1, characterized in that the period of circulation of the treating fluid is in the range of 11 hours to 13 hours.

15. A process according to claim 1, characterized in that the container (88) is purged with air.

16. A process according to claim 1, characterized in that the container (88) is purged with inert gas.

17. A process according to claim 1, characterized in that the membrane is comprised of a material having different permeation rates to different gases, thereby functioning as a gas separation membrane.

18. A process according to claim 3, characterized in that the membrane comprises a single layer of membrane material.

19. A process according to claim 3, characterized in that the module comprises multiple layers of membrane material.

20. A process according to claim 2, characterized in that the treating solution comprises one of n-heptane, isopropanol, and silicone.

21. A process according to claim 3, characterized in that the treating solution comprises one of n-heptane, isopropanol, and silicone.

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